How to make a Darlington pull down

Discussion in 'General Electronics Chat' started by rubicon99, Apr 27, 2012.

  1. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    I know how to make a Darlington pair pull up where you have two npn transistors and the emitter of the first is connected the the base of the second. And the emitter of the second ransistor is connected to ground. Both collecors are tied together with a load to power.

    Pretty much I am trying to create a on off switch with this darlington pull down. Can anyone give me some help with making the circuit.
     
  2. Audioguru

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    Dec 20, 2007
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    You described how to make an NPN darlington pull down which is what you want.
    It does not pull down to 0V because it has a saturation voltage loss of maybe 2V to 3V.

    Do you want a darlington to pull up instead? Then use a PNP darlington.
     
  3. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    Yes... how do you make a PNP darlington ... which ends go to supply and ground?
     
  4. Ron H

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    A MOSFET may be a better choice. How much current and voltage do you want to switch? Where does your switch input come from?
     
  5. rubicon99

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    Apr 19, 2012
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    switch input comes from a microcontroller. The max current is 100 mA. The input voltage is a ttl signal. The supply normally would be 12 volts, maybe stretching to 24 volts. I would like to keep it with transistors, even though i know that there will be more components being used.
     
  6. Ron H

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    You don't need a Darlington. You can do it like this.
     
  7. rubicon99

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    Apr 19, 2012
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    Thanks. So at the resistor load will I get the supply voltage if its on and ground if its off? About to breadboard it.
     
  8. Ron H

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    That's correct. A high signal (+3.3V to +5V) will turn the PNP on, connecting the supply voltage to the load. A low input signal (<0.4V) will turn off the PNP. This will remove the supply voltage to the load, but it will not connect ground to the high end of the load. The load will be floating on the high side, and will have zero volts across it.
     
  9. rubicon99

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    Apr 19, 2012
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    For some reason I am getting zero volts across the load. I must be doing something wrong.
     
  10. Ron H

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    Are you applying 5V to the input? Do you have the ground on your breadboard connected to the ground of your microcontroller, or whatever your input signal is coming from?
     
  11. rubicon99

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    Apr 19, 2012
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    I'm giving a high (measured 3.1) The ground is from the microcontroller... though I have other circuits on the board that are attached to the same power and ground. Probably not a good idea.... Also, so I can replace that circuit with one mosfet and get same results?
     
  12. Ron H

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    What is your load?

    If you are OK with the load always being connected to +V, and switching the ground side, you can use a singel logic-level NMOS FET. If you want to switch the high side, you could replace the PNP with PMOS FET. Some changes would be required in the rest of the circuit in this case.
    How fast are you switching the load?
     
  13. rubicon99

    Thread Starter Member

    Apr 19, 2012
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    I am applying a constant 3.3 (measured 3.1) volt input to the circuit and the ground is connected to the micro controller ground. The problem might be I havve too many of these small experimental circuits connected to the same ground and power on the breadboard
     
  14. rubicon99

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    Apr 19, 2012
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    So I can use a mosfet to achieve the same purpose?
     
  15. Ron H

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    See post #12.
     
  16. rubicon99

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    Apr 19, 2012
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    Thank you for the help.
     
  17. Ron H

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    If you need more troubleshooting or design help, we can do that.:)
     
  18. rubicon99

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    Apr 19, 2012
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    Instead of using the 2N2907 can I use the 2N3906. It seems that the pnp transistor is a more powerful amplifier. Just wondering so I can get a dual pnp and npn ic chip.
     
  19. rubicon99

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    Apr 19, 2012
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    Also will the pair that you showed me switch fast? ( hundreds of ns at the most )
     
  20. Ron H

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    I think we can do that with some modifications. I need more info first.
    1. What kind of microcontroller are you using?
    2. What is the vcc for that part?
     
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